c++: constant expressions are evaluated [PR93314]

My GCC 11 patch for PR93314 turned off cp_unevaluated_operand while
processing an id-expression that names a non-static data member, but the
broader issue is that in general, a constant-expression is evaluated even in
an unevaluated operand.

gcc/cp/ChangeLog:

	* cp-tree.h (cp_evaluated): Add reset parm to constructor.
	* parser.c (cp_parser_constant_expression): Change
	allow_non_constant_p to int.  Use cp_evaluated.
	(cp_parser_initializer_clause): Pass 2 to allow_non_constant_p.
	* semantics.c (finish_id_expression_1): Don't mess with
	cp_unevaluated_operand here.

3 files changed, 14 insertions, 16 deletions

diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index e80902a..d3639e3 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -5482,9 +5482,10 @@ class cp_evaluated
 public:
   int uneval;
   int inhibit;
-  cp_evaluated ()
+  cp_evaluated (bool reset = true)
     : uneval(cp_unevaluated_operand), inhibit(c_inhibit_evaluation_warnings)
-  { cp_unevaluated_operand = c_inhibit_evaluation_warnings = 0; }
+  { if (reset)
+      cp_unevaluated_operand = c_inhibit_evaluation_warnings = 0; }
   ~cp_evaluated ()
   { cp_unevaluated_operand = uneval;
     c_inhibit_evaluation_warnings = inhibit; }
diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index e1b1617..9603a1e 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -2163,7 +2163,7 @@ static enum tree_code cp_parser_assignment_operator_opt
 static cp_expr cp_parser_expression
   (cp_parser *, cp_id_kind * = NULL, bool = false, bool = false, bool = false);
 static cp_expr cp_parser_constant_expression
-  (cp_parser *, bool = false, bool * = NULL, bool = false);
+  (cp_parser *, int = 0, bool * = NULL, bool = false);
 static cp_expr cp_parser_builtin_offsetof
   (cp_parser *);
 static cp_expr cp_parser_lambda_expression
@@ -10374,13 +10374,15 @@ cp_parser_expression (cp_parser* parser, cp_id_kind * pidk,
   If ALLOW_NON_CONSTANT_P a non-constant expression is silently
   accepted.  If ALLOW_NON_CONSTANT_P is true and the expression is not
   constant, *NON_CONSTANT_P is set to TRUE.  If ALLOW_NON_CONSTANT_P
-  is false, NON_CONSTANT_P should be NULL.  If STRICT_P is true,
+  is false, NON_CONSTANT_P should be NULL.  If ALLOW_NON_CONSTANT_P is
+  greater than 1, this isn't really a constant-expression, only a
+  potentially constant-evaluated expression.  If STRICT_P is true,
   only parse a conditional-expression, otherwise parse an
   assignment-expression.  See below for rationale.  */
 
 static cp_expr
 cp_parser_constant_expression (cp_parser* parser,
-			       bool allow_non_constant_p,
+			       int allow_non_constant_p,
 			       bool *non_constant_p,
 			       bool strict_p)
 {
@@ -10416,6 +10418,11 @@ cp_parser_constant_expression (cp_parser* parser,
   parser->allow_non_integral_constant_expression_p
     = (allow_non_constant_p || cxx_dialect >= cxx11);
   parser->non_integral_constant_expression_p = false;
+
+  /* A manifestly constant-evaluated expression is evaluated even in an
+     unevaluated operand.  */
+  cp_evaluated ev (/*reset if*/allow_non_constant_p <= 1);
+
   /* Although the grammar says "conditional-expression", when not STRICT_P,
      we parse an "assignment-expression", which also permits
      "throw-expression" and the use of assignment operators.  In the case
@@ -24239,7 +24246,7 @@ cp_parser_initializer_clause (cp_parser* parser, bool* non_constant_p)
     {
       initializer
 	= cp_parser_constant_expression (parser,
-					/*allow_non_constant_p=*/true,
+					/*allow_non_constant_p=*/2,
 					non_constant_p);
     }
   else
diff --git a/gcc/cp/semantics.c b/gcc/cp/semantics.c
index 319a3a8..6224f49 100644
--- a/gcc/cp/semantics.c
+++ b/gcc/cp/semantics.c
@@ -4097,12 +4097,6 @@ finish_id_expression_1 (tree id_expression,
 
 	  cp_warn_deprecated_use_scopes (scope);
 
-	  /* In a constant-expression context, turn off cp_unevaluated_operand
-	     so finish_non_static_data_member will complain (93314).  */
-	  auto eval = make_temp_override (cp_unevaluated_operand);
-	  if (integral_constant_expression_p && TREE_CODE (decl) == FIELD_DECL)
-	    cp_unevaluated_operand = 0;
-
 	  if (TYPE_P (scope))
 	    decl = finish_qualified_id_expr (scope,
 					     decl,
@@ -4116,10 +4110,6 @@ finish_id_expression_1 (tree id_expression,
 	}
       else if (TREE_CODE (decl) == FIELD_DECL)
 	{
-	  auto eval = make_temp_override (cp_unevaluated_operand);
-	  if (integral_constant_expression_p)
-	    cp_unevaluated_operand = 0;
-
 	  /* Since SCOPE is NULL here, this is an unqualified name.
 	     Access checking has been performed during name lookup
 	     already.  Turn off checking to avoid duplicate errors.  */